1. Field of the Invention
The present invention pertains to sprayable, fiber-reinforced cementitious compositions which harden to form cementitious composites which exhibit strain hardening behavior.
2. Background Art
Sprayable cementitious compositions are known. Such compositions generally consist of a hydraulic cement such as Portland cement, a filler such as sand, water, and additives. The wet mortar is sprayed onto surfaces through a nozzle, to which it is conveyed by a pump, for example a spiral pump. Such conventional sprayable cement has noted disadvantages, however, in particular, substantially no ductility. In tension, only a very low percentage of strain can be tolerated before failure. Thus, such concrete is not suitable for applications where the concrete is expected to carry tensile loads, and even stresses caused by temperature changes can weaken the structure dramatically.
Sprayable cementitious materials, such as a concrete or mortar, can be defined as cementitious materials conveyed through a hose and pneumatically projected at high velocity from a nozzle into place. The rheological properties of the fresh mix in the wet spray processing are obviously crucial. The fresh mix should be moderately deformable, i.e., pumpable under the pumping and conveying pressure, so it will efficiently move through the hose to the nozzle. Once it is sprayed onto the surface of the substrates, however, it adhere to the substrate and remain cohesive without composite ingredient segregation.
Spray applications of fresh cementitious materials mix is regarded as beneficial for the repair of infrastructures such as bridges, culverts, underground structures, and other aged structures as well as for ground support such as a tunnel lining systems. Moreover wet process spray application has a number of advantages over cast, hand-applied, and dry spray processes, including the reduction or elimination of formwork, faster and more efficient construction, and reduction of environmental problems associated with heavy dust. For success of remedial work or ground support construction, the ductile performance of hardened cementitious materials is of great importance since many infrastructure deterioration problems and failures can be traced back to the cracking and brittle nature of cementitious materials. However, as indicated previously, conventional sprayable concrete or mortar lacks ductility.
In FIG. 1, the difference between brittle, quasi-brittle, and strain-hardening (ductile) behavior under uniaxial tensile load is portrayed schematically. In all three, tensile stress initially increases rapidly with strain, as shown in portion 1 of the portrayal. For brittle materials, catastrophic failure occurs, at relatively low strain, as shown by portion 2 of the Figure. In quasi-brittle behavior, a rapid decrease in stress with increasing strain also occurs, but limited and decreasing tensile strength is maintained for some period of increasing deformation (3). This deformation is localized in the form of a crack opening. In strain-hardening behavior, above a certain condition of stress/strain, tensile strength increases with increasing strain (4). Ultimately, failure will also occur, but at a much higher value of strain.
While the fracture toughness of sprayable concrete or mortar is significantly improved by fiber reinforcement, most fiber-reinforced sprayable concrete or mortar still exhibits quasi-brittle post-peak tension-softening behavior under tensile load where the load decreases with the increase of crack opening. The tensile strain capacity therefore remains low, about the same as that of normal concrete, i.e. about 0.01%. Significant efforts have been made to convert this quasi-brittle behavior of fiber reinforced concrete to ductile strain-hardening behavior resembling ductile metal. In most instances, the approach is to increase the volume fraction of fiber as much as possible. As the fiber content exceeds a certain value, typically 4–10% depending on fiber type and interfacial properties, conventional fiber reinforced concrete may exhibit moderate strain-hardening behavior. High volume fraction of fiber, however, results in considerable workability problem. Fiber dispersion becomes difficult because of high viscosity of the mix due to the presence of high surface area of the fibers and the mechanical interaction between the fibers, along with the difficulties in handling and placing. Moreover, the fibers in sprayable fiber reinforced concrete act as obstacles during conveyance through the pump and hose, resulting in excess pumping pressure. Thus, the fiber volume fraction is limited not only by workability considerations but also by pumpability considerations. While continuous fibers have been used in concrete to attain strain-hardening, the fabrication techniques utilized are totally incompatible with spray processing.
U.S. Pat. No. 4,057,528 discloses non-air-entraining mixes comprising Portland cement and a styrene-butadiene polymer. Considerable improvement in compressive strength is obtained. However, elongation at break is low and no strain-hardening behavior is disclosed.
U.S. Pat. No. 4,772,327 discloses sprayable concrete wherein sprayability is enhanced by incorporation of naphthalene sulfonate/formaldehyde condensates, setting of which may be accelerated by addition of alkali silicates or highly dispersed silica. The concrete is stated to exhibit superior adhesion to surfaces, but no indication of any improved strength properties or tensile ductility are disclosed.
U.S. Pat. No. 5,114,487 discloses sprayable concrete prepared from Portland cement, a ground sulfoaluminous clinker, calcium sulfate, aggregate, and small quantities of stainless steel fibers. The concrete is suitable for coating welds on sections of concrete-coated steel piping for transport of oil from off-shore oil platforms. The compositions are resistant to impact damage, but there is no disclosure of strength properties or tensile ductility.
U.S. Pat. No. 5,356,671 discloses sprayable compositions which include micro-silica pretreated with selected liquids to improve rebound properties of sprayed cement, particularly dry sprayed cement. Neither strength properties nor tensile ductility are discussed.
U.S. Pat. No. 5,413,819 discloses the use of phosphoric acid derivatives as stabilizers or “retarders” for use in sprayable cement compositions. No use of fiber reinforcement is disclosed, nor are any strength properties or tensile ductility discussed.
U.S. Pat. No. 5,609,681 discloses sprayable concrete to which polyoxyalkylene and β-naphthalene sulfonate-formaldehyde condensates are added as slump modifiers. No fibrous reinforcement is employed, and strength properties and tensile ductility are not disclosed.
U.S. Pat. No. 5,681,386 discloses sprayable two-component concrete mixes where a first reactive additive is added to the base concrete mix, optionally with steel and polypropylene fibers, and a second reactive component is mixed in at the point of spraying. The concrete remains pumpable and sprayable due to keeping the additives initially separate, but allowing them to react independent of the concrete mass. The process is complex, and no improvements in strength properties or tensile ductility are disclosed.
U.S. Pat. No. 5,961,712 discloses complex concrete mixtures which are suitable for spraying, and offer rapid setting and high early compressive strength. The mixtures contain gypsum and high aluminous content, preferably ground sulfo-aluminate clinker. The compositions are stated to provide properties similar to Portland cement-based compositions while offering decreased rebound. No mention of tensile ductility is made.
U.S. Pat. No. 5,993,537 discloses use of special polypropylene copolymer fibers which fibrillate during admixture to concrete mixes to provide concrete with enhanced impact resistance and flexural strength. However, improvement in uniaxial tensile strength or tensile ductility are not disclosed.